The following invention relates to an apparatus for forming flexible bags, a method for forming the flexible bag and the bag itself. More specifically, the following invention relates to bags to contain thermolabile and/or cellular biological substances and formed such that they can withstand stresses at extremely cold, cryogenic temperatures, reduce heat invasion from a higher ambient temperature into a plurality of such bags placed together, reduce storage space required for a plurality of such bags, and provide a thin and substantially constant cross-section for the bag so that the rate of heat transfer into and out of the bag is substantially homogenous throughout the contents of the bag thereby providing an improved means of protecting the viability of living cells during freezing and thawing.
Preservation of blood, cellular biological substances, tissue and other thermolabile products frequently involves product maintenance at extremely cold temperatures. Cellular biological substances are the fundamental, structural and functional unit of living organisms. Thermolabile substances are those substances which are easily altered or decomposed by heat. One economical mode for containment involves the use of encapsulating plastic since plastic is relatively inexpensive and lends itself to mass production techniques. However, many plastics suffer from brittleness at extremely low cryogenic temperatures and seams are sometimes susceptible to fracture.
In addition, bags that are formed either by folding over a planar material and seaming along peripheries or layering two planar materials and seaming along the peripheries have a generally ovoid shape when filled with a liquid. This is because the cross-sectional area adjacent either the fold or the seam has an area of decreasing cross-sectional width as it tapers from the center. While for many applications, this type of narrowing is unobjectionable, for certain biological fluids such as white stem cells, a bag having non-uniform thickness along its cross-section may impair the integrity of the biological product, particularly during temperature changes. One reason for quality loss during a change in temperature may involve the differential thermal gradient within the thermolabile or cellular substance caused by variations in thickness induced by the geometric shape of the bag itself. Stated alternatively, the center portion of the bag is thicker than the edges.
A corollary to the above-enunciated problem entails the fact that the prior art bags, with their thicker center portions, also provides a non-planar surface on opposing sides of the bag. This results in a xe2x80x9chigh spotxe2x80x9d which also makes uniform temperature alteration of the contents difficult especially when heat exchange is attempted by contact with a substantially planar surface that provides the heat gradient. Because the bag has a high area, uniform contact along the entire cross-section of the surface will have been precluded.
FIG. 8 reflects prior art bag structure and highlights the inherent problems associated therewith. The radio frequency seam S is thinner than the non-seamed plastic forming the bag and has its weakest point W at an edge of the seal closest to the interior I. When the product P begins to freeze, the product freezes first at the thinnest part of the bag, i.e. at edge E. Freezing proceeds inwardly, from the outside in, until an unfrozen core C exists. As the core C freezes, it expands and generates forces F which collimate and focus on the edge E because of the geometrical configuration of the bag. The force F frequently causes bag rupture at the weakest point W because the wedging force appearing at edge E tries to separate the seam with a turning moment M. Recall the bag material tends to become brittle at low temperatures, exacerbating this problem.
FIG. 13 reveals a further site of prior art bag weakness. When an access port tube T is to be fitted to the bag, two horseshoe-shaped RF horns H dose on the plastic membrane around the tube T and then the membrane at the seal area S. This causes another weakened area W where bag failures commonly occur.
The instant invention addresses all of the difficulties noted hereinabove. One manifestation of the problems solved includes the formation of peripheral seams circumscribing the bag which are appreciably stronger than prior art seam structures. Preferably, the bag is formed from two symmetrical sections, each vacuum formed to provide a major wall flanked by a radiused end wall section and circumscribed by a peripheral flange. It is contemplated in one form of the invention that two half portions defining a plane of symmetry are thus formed and adhered together so that the peripheral flanges are in mating registry and sealed such as by means of adhesive, ultrasonic, R.F. welding or other means. The increased surface area of the peripheral seam flange coupled with the radiused end wall interposed between the seam flange and the major wall of the bag dissipate forces which in prior art bags could have lent themselves to rupture at the seam area.
By vacuum forming the two halves, the contour of the side wall can be carefully controlled to make them substantially planar. Thus, when the two halves are united, a container having substantially uniform cross-section substantially along the entire extent will have been provided. This geometry encourages uniform thawing freezing and imperviousness to the stresses that attend the seams and the radiused end wall at cryogenic temperatures. This geometry also provides space efficient storage and reduced heat invasion from a warmer ambient medium when the plurality of bags are placed with their planar surfaces in contact with each other.
Accordingly, it is an object of the present invention to provide a useful and novel bag for containing thermolabile or live cell products, a method for making same and an apparatus associated with the formation of the bag.
A further object of the present invention is to provide a device as characterized above in which the seams associated therewith have greater strength to improve the integrity of the bag.
A further object of the present invention is to provide a device as characterized above in which the major surfaces of the bag are closely spaced, parallel and substantially planar thereby increasing the likelihood that heat transfer along those planar surfaces will more uniformly alter the temperature profile of the contents within the bag.
A further object of the present invention is to provide a device as characterized above which lends itself to mass production techniques, is extremely safe to use and is durable in construction.
A further object of the present invention is to provide a device as characterized above which is substantially rectangular so as to allow the device to be stacked side by side and end to end with minimal wasted space thereby achieving space efficient storage at low temperatures which results in a reduced operating cost.
A further object of the present invention is to provide a device as characterized above which has two parallel planar surfaces which make up a high percentage of the total exterior surface of the container such that when two or more such containers, lowered to subzero temperatures, are placed in contact with each other along these large planar surfaces, the percentage of the total surface area of the combined containers available to absorb heat from a surrounding warmer medium is substantially reduced.
Viewed from a first vantage point, it is an object of the present invention to provide a bag comprising, in combination: a first bag portion having a substantially planar outer wall, a radiused edge wall circumscribing the planar outer wall and peripheral flashing circumscribing the radiused edge wall, a second bag portion sealed to the peripheral flashing and overlying the planar outer wall of the first bag portion such that the planar outer wall is spaced from the second bag portion by a dimension at least equal to a radius of the radiused edge wall.
Viewed from a second vantage point, it is an object of the present invention to provide a method for forming a bag, the steps including: forming a first mold having a recess including a planar surface, a radiused periphery circumscribing the planar surface and a peripheral ledge circumscribing the radiused periphery and oriented parallel to the planar surface, placing a blank of sheet material over the first mold, and causing the blank to conform to the mold, removing the formed sheet and enclosing the bag.
Viewed from a third vantage point, it is an object of the present invention to provide a bag formed by: deforming a first sheet of material to have a planar outer wall, a radiused edge wall circumscribing the outer wall and peripheral flashing circumscribing the edge wall oriented parallel to the outer wall and defining a pocket, sealing the pocket with a second sheet of material.
Viewed from a fourth vantage point, it is an object of the present invention to provide a mold for forming bags comprising, in combination: a recess including a planar surface, a radiused periphery circumscribing the planar surface and a peripheral ledge circumscribing the radiused periphery and oriented parallel to the planar surface, means for receiving a blank of sheet material over the recess and lapped atop the peripheral ledge, and means for conforming the sheet material blank to a contour of the mold.
These and other objects will be made manifest when considering the following detailed specification when taken in conjunction with the appended drawing figures.